Surface enhanced Raman scattering or surface enhanced Raman spectroscopy (SERS) is a very sensitive and valuable analytical tool that enhances Raman scattering by molecules adsorbed onto or located on certain SERS active metal surfaces. The signal enhancement can be as high as 1014 or 1015, thus the method can be used to detect single molecules of interest. The exact mechanism of the enhancement is not currently known, although there are two prevailing theories. One theory, the electromagnetic theory, is that the enhancement results from excitation of localized surface plasmons. The chemical theory, on the other hand, attributes the enhancement to formation of charge-transfer complexes. The chemical theory, however, only applies to species that have formed a bond with the surface so it can not explain the enhancement in all cases whereas the electromagnetic theory is broader in application. Typical surfaces for SERS comprise particles or roughened surfaces of silver, gold, copper, palladium, or platinum. To get the SERS effect the surfaces must be rough, they can not be smooth. The surface can be prepared either by roughening a smooth surface of the metal or by depositing nanometer sized metal particles onto a surface. The shape and size of the nanoparticles and the thickness of the nanoparticle layer all effect the SERS signal. Because SERS requires a rough metal surface, conventional thin film growth techniques, which produce smooth thin films, are not suitable for forming SERS substrates.
One method for making a SERS substrate is to drop-cast or spread, a solvent, which contains nanoparticles, onto a surface; however, this method suffers from non-uniformity of the SERS active area, a lack of reproducibility, and inconsistent signal enhancement. Yet another method for formation of SERS surfaces is the chemical assembly method, which is also called the Langmuir-Blodgett (LB) method as disclosed in A. Tao et al., “Langmuir-Blodgettry of Nanocrystals and Nanowires” Ace. Chem. Res. Vol. 41. (2008) 1662-1673. In the LB method, noble metal nanoparticles are modified with hydrophobic molecules then dispersed in a volatile compound that is immiscible in water. This method is relatively simple and may be used on large-scale applications; however, the stability of the nanoparticle solution can be an issue.
In another method a substrate which provides a support material or a base is prepared by coating a patterned surface with a metal. First a pattern is imprinted into the surface. Then, SERS active metals are electrochemically or physically coated onto the patterned surface, to enhance the SERS signal as disclosed in B. Yan et al., “Engineered SERS substrates with multiscale signal enhancement: Nanoparticle cluster arrays” ACS Nano, Vol. 3. (2009) 1190-1202. This method requires several steps to obtain the desired SERS substrate and is typically expensive due to the required equipment used to produce the surfaces such as photo- or electron- lithography.
In yet another method a support material surface is first modified by laser nanomachining and then coated with a SERS active metal. For example, femtosecond laser processing has been used to prepare nanostructure surfaces which are then coated with reactive metals for SERS and other photonic sensing methods; however, the method still requires multiple steps to prepare the substrate as disclosed in U.S. Pat. No. 7,586,601. This method claims to be inexpensive compared with other patterned substrates based on lithography techniques discussed above, but it is still a multistep process and time consuming.
It is highly desirable to develop a method for producing a SERS active metal surface on a substrate that is inexpensive, rapid to carry out, highly reproducible, and tunable for detection of various substances.